Magnetic resonance imaging (MRI) may be used in conjunction with ultrasound focusing in a variety of medical applications. Ultrasound penetrates well through soft tissues and, due to its short wavelengths, can be focused to spots with dimensions of a few millimeters. As a consequence of these properties, ultrasound can be used for various diagnostic and therapeutic medical purposes, including ultrasound imaging and non-invasive surgery. For example, high-intensity focused ultrasonic waves (typically having a frequency greater than 20 kHz) may be used to therapeutically treat the diseased (e.g., cancerous) tissue without causing significant damage to surrounding healthy tissue.
An ultrasound focusing system generally utilizes an acoustic transducer surface, or an array of transducer surfaces, to generate an ultrasound beam. The transducer may be geometrically shaped and positioned such that the ultrasonic energy is focused at a “focal zone” corresponding to the target tissue mass within the patient. During wave propagation through the tissue, a portion of the ultrasound energy is absorbed, leading to increased temperature and eventually to cellular necrosis—preferably at the target tissue mass in the focal zone. The size and length of the focal zone generally depend on the ultrasound frequency, the focal depth, and the aperture size of the transducer. The individual surfaces, or “elements,” of the transducer array are typically individually controllable, i.e., their phases and/or amplitudes can be set independently of one another, allowing the beam to be steered in a desired direction and focused at a desired distance and the focal zone properties to be shaped as needed. Thus, the focal zone can be rapidly displaced and/or reshaped by independently adjusting the amplitudes and phases of the electrical signal input into the transducer elements.
In medical applications, the target location of the ultrasound focus is often determined using MRI. Generally, an MRI system, as depicted in FIG. 1A, includes a static-field magnet 102, one or more gradient-field coils 104, a radio-frequency (RF) transmitter 106, and an RF receiver (not shown). (In some embodiments, the same device is used alternately as RF transmitter or receiver.) The magnet includes a region 108 for receiving a patient 110 therein, and provides a static, relatively homogeneous magnetic field over the patient. The gradient-field coils generate magnetic field gradients that vary the static magnetic field. The RF transmitter 106 transmits RF pulse sequences over the patient to cause the patient's tissues to emit magnetic-resonance (MR) response signals. Raw MR response signals are sensed by the RF receiver and then passed to a computation unit 112 that computes an MR image, which may then be displayed to the user. MRI images provide radiologists and physicians with a visual contrast between different tissues and detailed internal views of a patient's anatomy that cannot be visualized with conventional x-ray technology.
The MRI system may be used to plan a procedure, for example, a surgical or minimally invasive procedure, such as a focused ultrasound ablation procedure, before its execution. A patient may initially be scanned in an MRI system in preparation for a procedure to locate a target tissue region and/or to plan a trajectory between an entry point (or region) and the target tissue region. Once the target tissue region has been identified, MRI may be used during the procedure, for example, to image the tissue region and/or to guide the trajectory of an external ultrasound beam to the target tissue region being treated. For example, using displayed images of an internal body region, a treatment boundary can be defined around the target tissue mass, and obstacle boundaries can be defined around tissue that should not be exposed to the ultrasound energy beam or whose exposure to ultrasound should be controlled and limited. The ultrasound transducer can then be operated based on these defined boundaries. These methods are generally referred to as magnetic-resonance-guided focused ultrasound (MRgFUS) methods.
While MRgFUS methods have been used effectively for the treatment of, for example, brain tumors and breast cancer, they still face significant challenges when applied to transcostal procedures, i.e., the treatment of visceral organs (such as the liver) that lie behind the rib cage. Ultrasound penetration of the rib cage does, in general, not only risk disruption of the acoustic beam profile by the ribs (resulting in diminished treatment efficacy for the target organ) and the formation of acoustic hot spots, but also undesired damage to the ribs. The location of the ribs, therefore, needs to be taken into account during treatment planning In rib images obtained using current conventional tomography (e.g., MRI), however, the boundaries of the ribs are usually difficult to identify due to low signal levels from the cortex and partial-volume effects, i.e., the presence of bone marrow and soft tissue in the same volumetric pixel (i.e., voxel). In addition, the use of an MRI apparatus for imaging places considerable constraints on the type of equipment that can be used in the system. For example, equipment constructed from ferromagnetic materials (such as motors, which may be desirable to use in a positioning system for the ultrasound transducer) cannot be used near an MRI system since the large magnetic fields generated by the MRI system will physically attract the magnetic equipment, and conductive materials can disturb and distort the RF electromagnetic fields necessary for resonance imaging.
Moreover, motion of the rib cage, e.g., during respiration, complicates rib localization. To precisely locate the ribs with respect to the ultrasonic transducer throughout the treatment, it may be necessary to track the rib cage continuously, and to stop the treatment process to correct for any misalignment due to a displacement of the rib cage and/or the internal organ to be treated. This results in significant inefficiencies in the treatment process and may generate significant delays.
Accordingly, there is a need for alternative methods of determining and tracking rib locations in an ultrasound treatment setting.